The present invention relates to an injection assembly for injection presses for plastic materials.
According to the prior art, in the injection moulding procedure injection presses or moulding devices are used in which resins, consisting of plastic materials, are heated in order to be brought to a molten state in a heating cylinder and are injected, at high pressure, into a mould cavity so as to fill the mould. The resin in the molten state is cooled inside the mould so as to harden and form the moulded product. At this point the mould is opened and the moulded product is removed and can be sent for storage or to subsequent manufacturing stages.
The injection moulding device comprises a moulding assembly and an injection assembly. The moulding assembly generally comprises two half-moulds mounted on a fixed plate and a moveable plate, respectively, so that the mould can be opened and closed by moving the moveable plate backwards and forwards.
FIG. 1 diagrammatically shows an injection assembly according to the prior art, denoted as a whole with reference numeral 100. The injection assembly 100 comprises a heating cylinder 2 to bring the resins fed through a hopper 3 to the molten state, and an injection nozzle 4 to inject the molten material into the mould cavity. Inside the heating cylinder 2 there is a plasticating screw 5 that makes a rotary movement around its own axis to plasticate the resins and a translational movement forward for injection of the molten material and backward to allow the resins to be introduced from the hopper 3.
The plasticating screw 5 is driven by means of a system of electric motors.
A screw and nut screw system is provided for the translational movement of the plasticating screw 5 wherein a screw 6 engages in a nut screw 7. The nut screw 7 is driven by a motor 8 by means of a reduction unit 9 secured to the frame 10 of the machine. The screw 6 is prevented from making a rotary movement around its own axis by means of an anti-rotation device. Said anti-rotation device comprises a rod 11 secured to the screw 6 and a stop block 12 secured to the frame 10 of the machine. In this manner the rod 11, by abutting against the stop block 12, prevents rotation of the screw 6, allowing only translation. Accordingly, operating the electric motor 8 in one direction or in the opposite direction permits feed or retraction of the screw 6 which pulls with it the plasticating screw 5.
The rotary movement of the plasticating screw 5 is obtained by means of a splined shaft 13, connected by means of a joint or coupling 20 to the plasticating screw 5. The splined shaft 13 engages inside a sliding bush or sleeve 14 which is rotated by an electric motor 15 by means of a reduction unit 16 secured to the frame 10 of the machine. The splined shaft 13 is uncoupled from the screw 6 by means of a bearing assembly 17, so that the of rotary movement of the plasticating screw 5 can be independent from the translational movement.
The electric motors 8 and 15 are equipped respectively with encoder type sensors 18 and 19 to carry out the speed adjustments required in the various stages of the work cycle.
The injection assembly 100, according to the above described prior art, has a drawback due to the fact that each electric motor 8 or 15 is dedicated to the translation stage or to the rotation stage of the plasticating screw 5. Therefore the power to be installed for each motor is equal to the maximum power required for the stage with which it is associated. Consequently, during the translation stage of the plasticating screw 5 overloading of the motor 8 will occur, while the motor 15 remains idle, whereas during the rotation stage of the plasticating screw 5, overloading of the motor 15 will occur, while the motor 8 remains idle. This leads to an excessive waste of energy and rapid wear on the motors which practically always work at maximum power.
The object of the invention is to eliminate these drawbacks by providing an electric injection assembly for injection presses for plastic materials that is practical, economical, versatile and easy to make.
In order to drive the plasticating screw, the electric injection assembly for injection presses for plastic materials according to the invention provides a single drive screw with two sections having threads with the helix in opposite directions. On each section of the drive screw operates a screw nut driven by a respective motor. The drive screw is connected to the plasticating screw by means of a coupling or joint. By suitably combining the speed of the two motors it is possible to carry out all the required stages of operation of the plasticating screw, that is to say rotation around its own axis, translation (feed and retraction), and rotary translation.
This system has various advantages with respect to systems of the prior art. In fact, in the injection assembly according to the invention it is possible, for any movement of the plasticating screw, to use all the power available from the two electric motors of the injection assembly, without using mechanical systems such as clutches, couplings or free-wheels.
Having identified the maximum power required for the various movements of the plasticating screw, the dimensions of each motor of the plasticating assembly will be such as to provide half of said maximum required power.
This system proves particularly simple and makes it possible to pass from one movement of the plasticating screw to another without a break, simply by changing the speed of the two motors.
Another advantage of the plasticating assembly according to the invention lies in the fact that the axial load operating on the screw during injection is divided between two nut screws instead of a single nut screw, as in the injection assemblies of the prior art, thus making the movement of the plasticating screw much more stable and balanced.